Sewage sludge from waste water treatment represents both a pollutant sink and also a storage means for nutrients such as nitrogen, phosphorus and potassium. There are therefore many different attempts at and methods of utilising sewage sludge as a valuable source of raw materials (‘secondary raw materials’). For example there are a series of methods of processing sewage sludge in order to be able to recycle it in agriculture as a phosphate fertiliser. On the other hand because of its high heavy metal and toxic substance loading sewage sludge is also viewed as critical as a fertiliser and is even treated as special waste. In some countries therefore agricultural sewage sludge exploitation is already limited or entirely stopped.
With that background in mind the recovery of reusable materials from the sewage sludge is of ever increasing significance in comparison with pure sewage sludge treatment and decontamination for further use. That concerns in particular the reusable material phosphorus, for which worldwide there is a high demand with at the same time limited availability. Phosphate is predominantly mined in the USA, China, Morocco and Russia. There are estimates that the phosphorus reserves which can be mined at reasonable cost and effort are sufficient only for between 60 and 130 years more. In that respect it is to be noted that it becomes increasingly difficult and also costly to acquire high-quality phosphorus ores which are only slightly contaminated with heavy metals.
Against the background of the limited worldwide reserves of phosphorus, to which it is possible to have recourse with present day mining conditions, and the rising price of raw phosphates and the prognoses about a consumption which is increasing having regard to the growth in population and future eating habits, new measures for obtaining and recovering phosphorus from other sources such as for example from the water or sewage sludge or sewage sludge ash are being increasingly discussed.
In the monoincineration of sewage sludge phosphorus remains as a residue in the ash. Depending on the mode of operation of the sewage treatment plant the concentration is between 4 and 8% by weight of P, or between 10 and 22% by weight of phosphorus pentoxide (P2O5). Further main components of the sewage sludge ash are SiO2 (30-50%), CaO (about 10-20%) and Al2O3 and Fe2O3.
Calcium is predominantly incorporated with the hardness of water. The silicon oxide originates from the incorporated solid materials such as sand, gravel and so forth. Aluminum compounds are in part incorporated by way of the water-softening zeolites contained in the washing agent.
In addition the sewage sludge ash contains heavy metals, inter alia Cr (50 ppm), Cu (350 ppm), Ni (30 ppm), Pb (10.0 ppm), Cd (1.3 ppm) and Hg (1.45 ppm). Organic toxic substances are generally destroyed without any residues by incineration of the sewage sludge and germs and odiferous substances are eliminated by incineration.
Various approaches are known in the state of the art for utilising or obtaining phosphorus from the ash from sewage sludge monoincineration processes.
1. Directly Applying the Ash to Agricultural Surfaces
That may be acceptable only when the heavy metal contents are very low and it can be demonstrated that the phosphorus is present in a plant-available form.
2. Direct Processing of the Ash in the Fertiliser Industry
This procedure may also be adopted only with a low level of heavy metal contamination as both quite a few heavy metals and also iron compounds cause problems in the processes used in phosphorus ore processing.
3. Washing out the Phosphates with Hot Water and Subsequent Precipitation or Crystallisation
In the present state of knowledge such phosphate recovery appears to be possible only with direct ashing of the excess sludge from increased biological P-elimination (bio-P-process). It is only on that condition that it is possible to succeed in recovering the phosphorus which is bound in the excess sludge in the form of polyphosphate after ashing of the sludge, in water-soluble form. Laboratory tests showed the applicability in principle of the method. It will be noted however that only the polyphosphate proportion which in present day conditions seldom makes up more than 40% of the raw phosphate payload can be recovered in that way. Added to that is the fact that, with stabilisation of the sewage sludges by digestion, which is usual in Germany, that involves rearrangement of the polyphosphate into a chemical-physical binding form which after ashing causes extraction with water to seem scarcely possible.
4. Elution of the Phosphates from the Ash with Sulfuric Acid
That process is offered by the Danish corporation PM Energi/BioCon A/S. In the BioCon process the phosphates are digested from the ash with sulfuric acid and eluted. Besides the phosphates iron and aluminum compounds as well as potassium are also extracted. In contrast ‘non-volatile heavy metals’ remain in the ash residue. Phosphorus is recovered as phosphoric acid by means of a battery of different kinds of ion exchangers. The heavy metals which are eluted at the same time occur as a specific fraction in concentrated form. The sulfate is recovered in the form of potassium hydrogen sulfate. The requirement for chemicals increases linearly with the precipitant content, for which reason the use of Fe or Al precipitants is to be reduced to the necessary minimum.
5. Krepro Process
In the Krepro process, a multi-stage process developed by Kemira Kemwater, Alpha Laval and the Helsingborg sewage treatment plant, the sewage sludge is separated into different products. In that case phosphorus is produced as iron phosphate. In the first stage the sewage sludge, after the addition of sulfuric acid, is heated at a pH-value of 1.5 and a pressure of about 4 bars to about 150° C. and hydrolysed. In that case a high proportion of the organic substance goes into solution. The undissolved proportion is then dewatered to 45% dry substance in a centrifuge and discharged. According to the description of the process the centrifugate contains the dissolved organic substances, the dissolved phosphorus, the precipitants and the redissolved heavy metals insofar as they are not bound to the sludge. After the addition of iron and stepwise increase in the pH-value to between about 8.5 and 9 iron phosphate (FePO4) precipitates, which in turn is separated by centrifuging from the liquid phase, thickened to about 35% dry substance and discharged as the product. In a further step, after renewed increase in the pH-value, the heavy metals are separated off—separately from the iron phosphate. What remains behind is a centrifugate, from which the precipitant iron oxide is also recovered, before it can possibly be used as a carbon source or has to be treated in the sewage treatment plant. The process is offered in two variants, as a continuous process and as a process which is operated batch-wise. The specific heavy metal content, that is to say the heavy metal content, in relation to phosphorus, of the iron phosphate produced, is to be present only at a fraction of the value of untreated sewage sludge and of a similar order of magnitude as in the case of mineral fertilisers. Energy consumption is high. The process however manages without energy from an external source if the sewage sludge is incinerated and used for energy production. Phosphorus recovery is about 75% of the amount introduced with the sludge.
6. Seaborne Process
The Seaborne process provides for joint treatment of sewage sludge in biogas installations for manure processing. It was developed by the Seaborne Environmental Research Laboratory and is intended to use various biomasses to produce the products fertiliser and methane gas in a pure quality which can be well used. Heavy metal sulfides in concentrated form and waste water occur as by-products. In the process, biomass is digested in a fermenter depending on the respective heavy metal loading either directly or after heavy metal digestion with H2S-bearing biogas. The digested biomass is dewatered in a separator. The solid is incinerated and the liquid phase is fed firstly to a heavy metal precipitation operation (RoHM=removal of heavy metals). Then the nutrients nitrogen, phosphorus and potassium are precipitated therefrom by various chemical precipitation reactions in the so-called NRS reactors (NRS=nitrogen recycling system). The H2S contained in the biogas is depleted in the RoHM reactors and used for the heavy metal precipitation process. The pre-purified biogas is freed from the CO2 in the gas scrubber referred to as the RGU (regenerative gas upgrading) so that almost pure methane (CH4>98%) occurs as the product. The CO2 contained in the biogas is used in the form of carbonate for precipitation of the nutrients in the NRS reactors. As the incineration ash is also recycled to the fermenter again via RoHM, there is apparently no solid waste except for the heavy metal salts which can be used in the electroplating art.
7. Phostrip Process
The Phostrip process is only limitedly comparable to the above-described procedures. Admittedly in this case phosphorus is recovered from the sludge, but only in such an amount as was additionally absorbed in the increased biological P-elimination procedure (‘luxury uptake’). The drawn-off excess sludge contains the same P-concentration as in the case of conventional processes so that recovery remains limited to between about 33 and 50% of the supplied sludge. The Phostrip process is to be viewed as a procedural variant in the bio-P-elimination process, in which redissolution of the phosphate is effected in the side-stream. A part of the return sludge is firstly fed to the pre-stripper and there mixed with organic substrate from the feed or the bottom discharge of the stripper in order to denitrify the nitrate contained in the sludge water under anoxic conditions. In the redissolution tank referred to as the stripper the sludge thickens under anaerobic conditions. In that case the phosphate stored in the cell is partially stripped from the biomass and discharged into the water phase. The sludge which is depleted in respect of phosphate is fed to the activation operation again and under aerobic conditions can again absorb and store phosphate. The orthophosphate-bearing supernatant is drawn off. In a precipitation reactor phosphate is precipitated with line milk or another precipitant at pH-values above 8.5 and then separated off. The process was installed in the two sewage treatment plants in Darmstadt (Germany) but stopped from time to time because of problems. It was reported that the precipitated calcium phosphate is almost free from organic impurities and P2O5 contents of between 33 and 41% are obtained in the dry material. Nonetheless for practicability reasons predominantly a sodium aluminate solution was used as the precipitant, which greatly limits the use of the phosphate. The heavy metal contents and AOX concentrations (AOX=absorbable organically bound halogens) in the precipitation sludge are to be very low (10% of the limit value of the Sewage Sludge Regulations for Cu, Zn and AOX and even lower for Cd, Cr, Hg, Ni and Pb). The process however presented problems in terms of controlling the procedure. With an inadequate residence time for the sludge in the stripper the P-redissolution was insufficient, with a sufficiently long residence time for formation of the organic acids and adequate P-redissolution, that involved severe hydrogen sulfide formation on the one hand and on the other hand damage to the sludge. In addition it is assumed there is a relationship between the proportion of thread-like micro-organisms in the activated sludge and operation of the Phostrip installation.
8. Ashdec Process
To be able to use sewage sludge ash, a process was developed in the past few years inter alia in the EU project ‘SUSAN’, by which the heavy metals, for example Pb, Cu, Cd, Zn and so forth are removed from the ash. The corporation Ash Dec—the name stands for ‘decontamination’ of ash—already has such a pilot plant in operation in Leoben, Austria. The process utilises the volatility of metal chlorides. The sewage sludge ash is mixed with environmentally compatible metal chlorides, generally CaCl2, compacted to form a granular material and heated in a rotary tubular kiln above the boiling point of the heavy metal chlorides which are formed, to between 900 and 1100 degrees. In that case the metal chlorides evaporate and are separated from the gas phase by flue gas scrubbers. Two products are obtained with that process, a sewage sludge ash which is suitable as a phosphorus fertiliser and whose heavy metal content is reduced by more than 90% in relation to the original content, and as a further product, a residue with a high metal concentration which in future could also be put to economic use. In particular aluminum, iron, high-quality steel or copper can be obtained from the residue.